Give comparison between the following: (a) C₃ and C₄ pathways (b) Cyclic and non-cyclic photophosphorylation (c) Anatomy of leaf in C3 and C4 plants

(a) C₃ and C₄ pathways

C₃ pathways:-
1. The primary acceptor of CO₂ is RUBP – a six-carbon compound.
2. The first stable product is 3phosphoglycerate.
3. It occurs only in the mesophyll cells of the leaves.
4. It is a slower process of carbon fixation and photo-respiratory losses are high.
C₄ pathways:-
1. The primary acceptor of CO₂ is phosphoenol pyruvate – a threecarbon compound.
2. The first stable product is oxaloacetic acid.
3. It occurs in the mesophyll and bundle-sheath cells of the leaves.
4. It is a faster process of carbon fixation and photo-respiratory losses are low.

(b) Cyclic and non-cyclic photophosphorylations
Cyclic photophosphorylation:-
1. It occurs only in photosystem I.
2. It involves only the synthesis of ATP.
3. In this process, photolysis of water does not occur. Therefore, oxygen is not produced.
4. In this process, electrons move in a closed circle.
Non-cyclic photophosphorylation:-
1. It occurs in photosystems I and II.
2. It involves the synthesis of ATP and NADPH₂
3. In this process, photolysis of water takes place and oxygen is liberated.
4. In this process, electrons do not move in a closed circle.

(c) Anatomy of the leaves in C₃ and C₄ plants
C₃ leaves:-
1. Bundle-sheath cells are absent
2. RuBisCo is present in the mesophyll cells.
3. The first stable compound produced is 3-phosphoglycerate –a three-carbon compound.
4. Photorespiration occurs
C4 leaves:-
1. Bundle-sheath cells are present
2. RuBisCo is present in the bundlesheath cells.
3. The first stable compound produced is oxaloacetic acid – a four-carbon compound.
4. Photorespiration does not occur

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